It is well known in the field of thermoforming plastics and more particularly in container manufacture to utilize a composite or "sandwich" sheet of thermoplastic material manufactured through the process of coextrusion whereby a first polymeric material is coextruded onto a second polymeric material and adhered thereto by means of a "tie" layer of a third coextruded polymeric material. For example, it has been known to utilize layers of polyethylene terephthalate (PET) and ethylene vinyl alcohol (EVAL) t manufacture containers having high barrier characteristics and being impervious to chemical attack. Such composite materials are coextruded through sheet coextruders wherein the final sheet may have a layer of PET sandwiched on each side of a layer of EVAL. Other known coextruded composite sheets include combinations of the known thermoplastic polymers such as polystyrene, polyethylene, and polypropylene, and their various related homologues.
Other technologies attempt to obtain the beneficial properties of various polymer materials by "blending" the materials into an "alloy" which hopefully retains the beneficial qualities of each particular polymer while overcoming the deficiencies of those polymers. One example of such a blend is that disclosed in U.S. Pat. Nos. 4,386,187 and 4,386,188 which teach a blend of polyolefin such as polyethylene or polypropylene as the major component with a styrene polymer as a secondary component and a "suitable compatibilizer". The compatibilizer utilized comprises mainly the SBS (styrene-butadiene-styrene) rubbers and the styrene-isoprene and styrene-ethylene rubbers. The blends consist of a compounded mixture of the olefin material, the styrene polymer, and the SBS rubber, which result in an "alloy" having a translucent or pearlescent color and hopefully retaining the beneficial properties of the blend components.
Another blended alloy is that disclosed in U.S. Pat. No. 3,810,957, consisting of a monovinyl aromatic compound such as polystyrene, blended with a block copolymer of a monovinyl arene and a selectively hydrogenated diene.
Still another alloy is that disclosed in U.S. Pat. No. 4,031,166 consisting of polystyrene and a "soft component" which consists of a mixture of a polyolefin and a copolymer of a vinyl aromatic and a diene hydrocarbon.
The difficulties incurred with these types of blends is that the compatibilizer (SBS rubber) which is utilized therein, does not offer sufficient compatibility to substantially eliminate the phenomenon of delamination which occurs between the various component particles in the blend. The SBS compatibilizer allows the olefinic material to delaminate from the styrenic polymer material and produces powdering and flaking of the final blended thermoformed product. Also, the known processes and blends require a usage of a very large proportion of the olefinic material compared to the styrenic polymer which adds to the cost of the final product.